High definition (HD) videos and HDTV broadcasting become more popular in the entertainment industry. The HD videos and HDTV contents are generally recorded in an optical disc. The conventional optical drive for writing and playing back the HD videos and HDTV generally comprises a house, a disc/spindle assembly, an optical pickup assembly and a plurality of control circuits, in which the optical pickup assembly is a “heart” of the optical drive and is used for reading from and writing data into the optical disc.
The optical pickup assembly mainly comprises a laser generator (i.e. laser diode), a half-reflecting prism, an objective lens and a plurality of photodiodes. When reading the data from the optical disc, the laser generator generates and emits laser beams. The generated laser beams run through the half-reflecting prism and are converged into the objective lens, which in turn focuses the converged laser beams into small optical spots and sends the optical spots to the disc. Reflecting materials on the disc will reflect the optical spots. The reflected optical spots are transmitted through the objective lens and then reach the photodiodes via the half-reflecting prism. Since there are some pits in a surface of the disc to record the data, the laser beams are reflected from the surface of the disc in various directions. Signals of the laser beams in various directions may be represented as “0” or “1”. The photodiodes decode the data of “0” or “1” in a desirable format for playing back.
In the optical disc, data are recorded in concentric spiral tracks. When reading the data from the concentric spiral tracks, the error correction capability and reliability of the optical pickup assembly will directly depend on two motions of “tracking” and “focusing”. The “tracking” is to keep the laser beams aiming at the concentric spiral tracks. The called “focusing” is to accurately transmit the laser beams to the disc with the strongest reflected signal from the disc. The objective lens may be moved in a vertical direction to provide the motion of “focusing”, and moved in a horizontal direction to provide the motion of “tracking”. In order to maintain the “tracking” and “focusing” motions, a continuous adjustment of the lens-disc separation and of a radial position on the track are performed when reading and writing.
Currently large capacity optical disc formats such as Blu-ray and HD-DVD have been proposed. Accordingly, the optical drive is needed to be improved so as to quickly and precisely position optical detecting spots in tracking and focusing motions in optical pickup assembly.